Question

For a series of indicators, the colours and pH range over which colour change takes are as follows:

Indicator	Colour change over pH range
U	Yellow to blue pH 0.0 to 1.6
V	Red to yellow pH 2.8 to 4.1
W	Red to yellow pH 4.2 to 5.8
X	Yellow to blue pH 6.0 to 7.7
Y	Colourless to red pH 8.2 to 10.0

Which of the following statements is/are correct?

• A. Indicator V could be used to find the equivalence point for $0.01M$ ammonium hydroxide (ammonia solution) titration with 0.01 M $C{H}_{3}COOH$ solution
• B. Indicator Y could be used to distinguish between $0.1M$ $HCl$ and $0.001M$ $NaOH$ solutions in water
• C. Indicator X could be used to distinguish between solutions of ammonium chloride and sodium acetate
• D. Indicator W could be suitable for use in determining the concentration of acetic acid in vinegar by base titration

Answer 1

Answer: (B), (C)

The correct options are
B Indicator Y could be used to distinguish between $0.1M$ $HCl$ and $0.001M$ $NaOH$ solutions in water
C Indicator X could be used to distinguish between solutions of ammonium chloride and sodium acetate

a) For the titration of weak acid with weak base, there is no sharp change in the $pH$ value near the end point ($pH$ varies between 6.0 and 8.0).
$C{H}_{3}COOH+N{H}_{4}OH$ $\to$ $C{H}_{3}COON{H}_4$ $pH=7.0$
So indicator V cannot be used. But indicator X can be used.

b) Y can be used since it will give different colour in both the solutions.
$0.1MHCl$ has $pH=1.0$
$0.001MNaOH$ has $pH=11.0$

c) X can be used since it will give different colour in both the solutions.
$N{H}_{4}Cl$ (salt of weak base + strong acid $pH<7.0$ ) and
$C{H}_{3}COONa$ (salt of weak acid and strong base $pH>7.0$ )

d) Y will be suitable indicator not W.
$C{H}_{3}COOH+base$ $\to$ $pH>7.0$